With a view to their use as biofuel, vegetable oil alkyl esters are produced from vegetable oils obtained for example from rapeseed, sunflower, soybean or even palm. III-suited for directly feeding modern diesel engines of private cars, vegetable oils essentially consisting of triglycerides have to be converted by means of a transesterification reaction with an alcohol, methanol or ethanol for example, introduced in excess to produce vegetable oil methyl esters (VOMEs) and glycerin.
The current EN 14,214 European standard on biofuels sets maximum methanol, water, free glycerol, mono-, di- and triglyceride contents: 0.2% by mass methanol, 500 mg/kg water, 0.02% by mass free glycerol, 0.8% by mass monoglycerides, 0.2% by mass di- and triglycerides.
Free glycerol, as opposed to bonded glycerol, is defined as a glycerol molecule totally detached from any carbon chain and of formula C3H8O3.
Glycerol is referred to as bonded when the functional group of glycerol C3H8O3 is alkylated to one or more fatty acid chains giving monoglyceride, diglyceride or triglyceride molecules.
The vegetable and/or animal oils used in this alkyl ester manufacturing process can be any oils known to the person skilled in the art, for example rapeseed, palm, sunflower, soybean, copra, castor oil, as well as fatty substances of animal origin such as tallow.
The alcohol used generally is an aliphatic monoalcohol. Preferably, the alcohol essentially consists of methanol and/or ethanol.
Glycerol is known to be infinitely soluble in water and alcohols, little soluble in ethers and totally insoluble in benzene (Handbook of Chemistry and Physics, 54th Edition, 1973-1974), i.e. it is soluble in rather polar media. Glycerol and esters such as methyl esters have very little mutual solubility but methanol acts as a co-solvent. The glycerol content of the ester phase increases with higher temperatures and higher methanol contents.
In order to reach the content allowed by the fuel specification (below 200 ppm), it is necessary to separate the glycerin dissolved in the ester phase.
Depending on the type of method used, this separation is achieved differently. In the methods referred to as homogeneous catalysis methods, as described for example in document EP-B1-0,356,317 or WO-2007/034,067, the purification stage can be carried out through successive water wash operations and passage through ion-exchange resins.
In the Esterfip-H™ process developed by IFP and using a heterogeneous catalyst, separation between the glycerol and the ester produced occurs in several stages. In fact, methanol acting as a co-solubilizing agent for the methyl esters and the glycerol, the alcohol evaporation stage carried out at the reaction section outlet makes part of the glycerol present in the stream, in a proportion ranging between 0.1 and 5% by mass, insoluble. The soluble part represents, at ambient temperature, 500 to 700 ppm mass, the allowable maximum content set by the European standard being 200 ppm mass of free glycerol. Both the insoluble glycerol and part of the soluble glycerol therefore have to be separated. Separation is first performed through gravity decantation in a decanter drum. The ester stream flowing from the decanter is then sent to a coalescer. The glycerin phase stream is withdrawn at the bottom point of the coalescer. Methods allowing these two separation stages to be improved are described in the patent applications filed by the assignee under No. FR-07/04,711 and FR-07/04,715.
It is however necessary to perform a final purification stage by adsorption on solids, ion-exchange resins for example. At the end of this stage, the glycerin content of the ester phase thus meets the fuel specification (below 200 ppm).
The ion-exchange resins used in these stages of final purification of the ester phase operate by alternating adsorption and regeneration cycles.
The adsorbent solids thus are in contact with part of the insoluble glycerin. The properties of these solids are such that they have a very high affinity towards the glycerin contained in the stream to be purified. When this stream predominantly containing ester, partly converted glycerides, methanol traces and soluble and insoluble glycerin passes through the fixed resin bed, the solid captures, i.e. retains, through a “physisorption” phenomenon, the soluble and insoluble glycerin and allows the other molecules to pass through. This solid has sites that attract and retain the glycerin because of their configuration and/or polarity. Each site can however capture only a finite number of glycerin molecules, the other ones present in the ester stream will pass through without being retained. Thus, a glycerin molecule present in the stream to be purified flowing through the bed will be captured by the first unsaturated site it encounters, but it will pass through the bed if it encounters none. Globally, as long as the bed is not saturated, there will be no glycerin in the stream obtained at the resins outlet, but as soon as the bed is saturated, the glycerin content in this stream will increase and become identical to that of the incoming stream to be purified (breakthrough phenomenon). It is then necessary to regenerate the resins, i.e. to remove the glycerin captured by a suitable solvent.
In order to permanently ensure a stream produced without glycerin, meeting the biofuel specifications imposed by the current European standard, the purification zone consists of two adsorbers operating intermittently. While one is in the adsorption phase, the other is regenerated so as to always have a regenerated adsorber when the one working in adsorption mode becomes saturated.
For the specification relative to the glycerin content in the final ester stream to be continuously met, it is thus essential to very finely monitor the breakthrough time, i.e. the time when the bed is saturated and therefore when the glycerin content of the stream produced increases very rapidly and might exceed the glycerin content allowed by the fuel specification. The breakthrough time, i.e. the elapsed time between a quasi-zero glycerin content and the content of the incoming stream, is of the order of 30 minutes in the case of the Esterfip-H™ process.
There are analysis methods that are commonly used to determine the glycerin content of the outgoing stream and therefore to detect the breakthrough time. Gas chromatography can notably be used. This method, described in the EN 14,214 standard, requires a sample preparation of 15 to 30 minutes minimum prior to injection in a chromatogram, then the chromatographic analysis proper is carried out and lasts 2 hours on average.
Another method consists in carrying out glycerin determination with metaperiodate. This reaction is based on a chemical reaction with metaperiodate under controlled pH, one of the reactions products being then determined. This reaction is conducted in the aqueous phase. A water wash operation therefore has to be carried out on the ester sample so as to extract all of the glycerin present, and glycerin determination is performed after decanting in the aqueous phase. Even if determination can be automated, the sequence of these operations (washing+decanting+determination) penalizes the accuracy of the method, which furthermore remains long (between 1 h30 and 2 hours).
The methods and the analysis times currently used are not compatible with fine monitoring, i.e. real-time detection of the breakthrough of the resins and, more generally, of the adsorbent beds. In fact, this involves multiplying costly analyses and, at best, changing adsorbers too late after breakthrough (1 h30 to 2 h thereafter), which implies the production of a quite considerable volume of an off-specification product.
A method allowing detection and/or very fast monitoring, in real time, of the presence of glycerin in the ester phase is therefore needed.
The present invention provides an adsorbent bed breakthrough detection and/or monitoring method that does not involve the aforementioned drawbacks and applies to biodiesel manufacturing processes wherein the glycerin has to be separated from the product obtained.